Passive phasing of fiber amplifiers

ABSTRACT

A method passively locks and phases an array of fiber amplifiers in a fiber amplifier system that emits a beam, such as a laser beam. The method locks the fiber amplifiers so that the fiber amplifiers operate at same or similar frequencies. The method samples a small portion of the emitted beam in a far-field around a central lobe on an optical axis and then couples this portion of emitted beam back into the array of fiber amplifiers. The fiber amplifiers may be phased so that the emitted beam concentrates its energy around the central lobe in the far-field. Phasing may be achieved by using an aperture, for example, to restrict the portion of the emitted beam to be coupled back to a restricted region around the optical axis.

TECHNICAL FIELD

The technical field relates to fiber amplifiers, and, in particular, topassive locking and phasing of an array of fiber amplifiers.

BACKGROUND

High power solid-state lasers, for military and industrial applications,have been the focus of vigorous recent research. Several currentapproaches include slab lasers and heat capacity lasers for bulksolid-state laser systems, and coherent fiber laser arrays for fiberbased laser systems. The major problem confronting methods employingbulk solid-state materials used in bulk solid-state laser systems isheat management. Thermal gradients cause the laser beam to deterioratein quality. In addition, system efficiency deteriorates due to hightemperature's negative impact on the inversion process. The majorproblem confronting the fiber based laser systems is the complexityassociated with actively (i.e., electro-optically) phasing a largenumber of single-mode fiber amplifiers employed in fiber phasing,especially when the number of fiber amplifiers is large. Employingsingle-mode fiber amplifiers in these fiber amplifier arrays isnecessary to preserve good beam quality for the total system output.However, power scaling in single-mode fiber amplifiers is limited incurrent solutions.

SUMMARY

A method for passive phasing of fiber amplifiers includes placing anoptical device at a far-field of an array of fiber amplifiers andemitting a beam from the array of fiber amplifiers. The method furtherincludes coupling a first portion of the emitted beam back into thearray of fiber amplifiers through a coupling hole. Only the firstportion of the emitted beam propagates through the coupling hole.

A corresponding system for passive phasing of fiber amplifiers includesan array of fiber amplifiers including a plurality of fiber amplifiers.The array of fiber amplifiers emits a beam. The system further includesan optical device placed at a far-field of the array of fiberamplifiers, wherein the optical device couples a first portion of theemitted beam back into the array of fiber amplifiers through a couplinghole. Only the first portion of the emitted beam propagates through thecoupling hole.

In one exemplary embodiment,.the optical device is a mirror positionedin the far-field of the array of fiber amplifiers, the optical device isplaced around a central lobe on an optical axis of the an array of fiberamplifiers, and only the first portion of the emitted beam that isin-phase propagates through the coupling hole. In another exemplaryembodiment, the optical device is a collimating mirror that couples thefirst portion of the emitted beam to an array of fiber pre-amplifiersbefore coupling the first portion of the emitted beam to the array offiber amplifiers. In yet another exemplary embodiment, the opticaldevice is a beamsplitter that redirects the first portion of the emittedbeam to a tuning grating. In still another exemplary embodiment, theoptical device is a beamsplitter enclosed in an input head. The inputhead also includes a second harmonic generator (SHG) crystal forfrequency doubling.

DESCRIPTION OF THE DRAWINGS

The exemplary embodiments of the method and system for passive phasingof fiber amplifiers will be described in detail with reference to thefollowing figures, in which like numerals refer to like elements, andwherein:

FIGS. 1-6 illustrate exemplary embodiments of a fiber amplifier systemfor passive phasing of fiber amplifiers;

FIG. 7 illustrates an exemplary experimental far-field intensity profilefor the exemplary embodiment of FIG. 4 of the fiber amplifier system;

FIG. 8 illustrates a comparison of theoretical and experimentalintensity profile measurement for two coupled fiber amplifiers;

FIG. 9 schematically shows exemplary resonant modes with the highestfeedback in locking and phasing the fiber amplifiers; and

FIG. 10 is a flow chart illustrating an exemplary method for passivephasing of fiber amplifiers.

DETAILED DESCRIPTION

A method passively locks and phases an array of fiber amplifiers in afiber amplifier system that emits a beam, such as a laser beam. Themethod locks the fiber amplifiers so that the fiber amplifiers operateat same or similar frequencies. The method samples a small portion ofthe emitted beam in a far-field around a central lobe on an optical axisand then couples this portion of emitted beam back into the array offiber amplifiers. The fiber amplifiers may be phased so that the emittedbeam concentrates its energy around the central lobe in the far-field.Phasing may be achieved by using an aperture, for example, to restrictthe portion of the emitted beam to be coupled back to a restrictedregion around the optical axis. The far-field typically collects theoutput of all the fiber amplifiers. Coupling a portion of this far-fieldemitted beam back into the fiber amplifiers ensures strong coupling ofall the fiber amplifiers because each fiber amplifier is effectivelycoupled to all other fiber amplifiers in the fiber amplifier system.This method thus leads to a more robust coupling.

When the output of all of the fiber amplifiers are in-phase, the fiberamplifier system is in an in-phase mode, i.e., longitudinal mode. Thein-phase mode typically has the highest intensity around the centrallobe on the optical axis in the far-field. On the other hand, anout-of-phase mode (where the output of the fiber amplifiers areout-of-phase) typically has zero intensity on the optical axis. Theintensity for the in-phase mode on the optical axis may be proportionalto N², where N is the total number of fiber amplifiers in the array. Thefar-field distance may be a few decimeters, which is a short distance sothat no additional optics are required to transmit the emitted beam tothe far-field.

A typical fiber amplifier has a relatively broad gain line-width and cansupport hundreds of thousands of longitudinal modes. The method cantherefore find one or more longitudinal modes, referred to as resonantmodes, within the gain line-width that result in perfect phasing ofnon-identical fiber amplifiers. In addition, the method automaticallymay correct mechanical and thermal perturbations by rapidly (withinmicroseconds) adapting to a new longitudinal mode, thus ensuring thephasing of the fiber amplifiers. Furthermore, the fiber amplifiers maybe preceded by fiber pre-amplifiers to enhance the feedback signal.Therefore, only a small portion, such as a few percent, of the emittedbeam needs to be coupled back into the fiber amplifiers. Most of theemitted beam may be passed through to the output. As a result, the lossdue to feedback is low, leading to an efficient fiber amplifier system.

FIG. 1 illustrates an exemplary embodiment of a fiber amplifier system100 for passive phasing of fiber amplifiers. The system 100 includes anarray of fiber amplifiers 110 connected to an array of microlenses 120,such as collimating microlenses. The fiber amplifier array 110 emits abeam 182, such as a laser beam, using the microlens array 120.

The system 100 further includes a feedback reflector 150 to couple aportion of the far-field emitted beam 182 back into the fiber amplifierarray 110. The coupled back portion, referred to as a coupled back beam184, may be ten or twenty percent of the emitted beam 182. The feedbackreflector 150 may be a meniscus lens with, for example, a ten or twentypercent reflectance to reflect the emitted beam 182. The system 100 mayinclude an iris 140 that blocks most of the energy except for thecentral lobe. While the different embodiments of the fiber amplifiersystem is described using in-phase modes, those skilled in the art willreadily appreciate that the fiber amplifier system can be applied to alltypes of passive phasing mechanism, regardless of the selection ofin-phase or out-of-phase modes.

The fiber amplifier array 110 may form double-pass amplifiers byterminating each fiber amplifier with a high reflectivity fiber Bragggrating (FBG) (e.g., see FBG 270 in FIG. 2). FBG deflects beamscompletely. Referring to FIG. 1, the coupled back beam 184 is coupledback to the fiber amplifier array 110 through a coupling hole 130, i.e.,an aperture, in a cooling plate 160. The rest of the emitted beam 182,i.e., the portion that is not reflected back into the fiber amplifierarray 110, is transmitted through the feedback reflector 150 as anoutput beam 180. Additional optics well known in the art may be used tocollimate the output beam 180.

FIG. 2 illustrates a second exemplary embodiment of the fiber amplifiersystem 200 for passive phasing of fiber amplifiers. The system 200 issuitable for high power scaling. The system 200 includes an array offiber amplifiers 210 connected to an array of collimating microlenses220. The fiber amplifier array 210 emits a beam 282, such as a laserbeam, using the microlens array 220. The fiber amplifier array 210 mayform double-pass amplifiers by terminating each fiber amplifier with ahigh reflectivity FBG 270.

The system 200 further includes two mirrors 250, 260. The mirror 250 inthe far-field couples a portion the emitted beam 282 back into the arrayof fiber amplifiers 210 through a small coupling hole 230 in the primarymirror 260. The coupled back portion is referred to as a coupled backbeam 284. The two mirrors 250, 260 form a telescope 255, generating acollimated output beam 280 from the portion of the emitted beam 282 notreflected back into the fiber amplifier array 210.

With continued reference to FIG. 2, the emitted beam 282 typically hasgood beam quality in the far-field and is not effected adversely by thecoupling hole 230. In effect, the system 200 not only filters outtransverse modes, i.e., out-of-phase modes where the fiber amplifiersare not in-phase, but also filters out longitudinal modes that do notlead toga transverse distribution where all the emitted beams 282 arein-phase. The system 200 is not sensitive to the non-uniformity of fiberamplifier dimensions or separation between the fiber amplifiers.Therefore, the manufacturing tolerances for subcomponents of the fiberamplifier system can be relaxed.

The system 200 is also suitable for scaling to high power levels, suchas multi-kilowatts, because the output beam 280 is not transmittedthrough a lens, such as the feedback reflector 150 in FIG. 1.Transmitting light through a lens may introduce aberrations due tothermal effects in the lens. Therefore, the system 200 is scalable inpower, as well as number of fiber amplifiers. Moreover, the output beam280 produced by the system 200 is automatically collimated withoutadditional optics, reducing the number of subcomponents of the fiberamplifier system.

FIG. 3 illustrates a third exemplary embodiment of the fiber amplifiersystem 300 for passive phasing of fiber amplifiers. The system 300 usesa ring configuration. In the system 300, two ends 322, 324 of an arrayof fiber amplifiers 310 are positioned towards each other along anoptical axis. The fiber amplifiers array 310 is unidirectional inconstruction because each fiber amplifier has an internal isolator 390that allows beams to flow in one direction only (the direction of thearrow, for example). The fiber amplifier array 310 may be connected toan array of microlenses 320 at a transmitting end 322. The fiberamplifier array 310 may emit a beam 382 of high energy using themicrolens array 320.

The system 300 further includes a curved collimating mirror 360 thatintercepts a coherent sum of the emitted beam 382 from the fiberamplifiers array 310. The collimating mirror 360 collimates and reflectsmost of emitted beam 382 as a collimated output beam 380 except for asmall portion, such as two or three percent. The collimating mirror 360has a coupling hole 330, which allows the small portion of the emittedbeam 382, referred to as a coupled back beam 384, to be coupled backinto a receiving end 324 of the fiber amplifier array 310. The size ofthe coupling hole 330 may be as large as the receiving end 324.

With continued reference to FIG. 3, at the receiving end 324, an arrayof microlenses 328 receives and focuses the coupled back beam 384 intoan array of fiber pre-amplifiers 315. The coupled back beam 384typically has weak signals, which are amplified by the fiberpre-amplifier array 315 before being coupled into the array of fiberamplifiers 310. The microlens array 328 may be an array of tiny lenses.Each lens may have a diameter of approximately two hundred and fiftymicrons and may collect and focus the coupled back beam 384 into one ofthe fiber pre-amplifiers, for example. One microlens typicallycorresponds to one fiber amplifier or fiber pre-amplifier.

The in-phase mode typically has the highest intensity on the opticalaxis. The out-of-phase mode typically has zero intensity on the opticalaxis. Therefore, the in-phase mode may have the highest feedback and thelowest loss on the optical axis. The distance L 350 between the two ends322, 324 of the fiber amplifier array 310 may be large enough to allowthe coherent superposition of the emitted beam 382 to form a welldefined central lobe for the in-phase mode. This length L 350 may be inthe Rayleigh range, i.e., the range where the beam is basicallycollimated, for the transmitting fiber amplifiers.

The system 300 uses the low powered fiber pre-amplifier array 315 toincrease the intensity of the optical signal, i.e., coupled back beam384, before coupling the coupled back beam 384 into the high poweredfiber amplifier array 310. Consequently, only a small portion, such astwo or three percent, of the emitted beam 382 is needed for feedback.The system 300 is therefore more efficient. In addition, noises, such asamplified spontaneous emission (ASE), may be blocked by the isolator390. The isolator 390 may also block stimulated Brilloumn scattering(SBS) because SBS typically propagates in the opposite direction.

FIG. 4 illustrates a fourth exemplary embodiment of the fiber amplifiersystem 400 for passive phasing of fiber amplifiers. The system 400 usesa tuning grating 495 that enables frequency tuning. Compare with thesystem 300, the system 400 separates and positions a transmitting end422 and a receiving end 424 of an fiber amplifier array 410 on a sameside without facing each other. The fiber amplifier array 410 isconnected to an array of microlenses 420 at the transmitting end 322.The fiber amplifier array 410 emits a beam 482 of high energy using themicrolens array 420.

The system 400 further includes a beamsplitter 485 that redirects theemitted beam 482. The beamsplitter 485 may redirect a small portion,such as four percent, of the output beam 482 through an aperture 430.The small portion is referred to as a coupled back beam 484. The rest ofthe emitted beam 482 propagates through the beamsplitter 485 as anoutput beam 480. In the system 400, only the portion of the coupled backbeam 484 with in-phase mode propagates through the aperture 430 to thetuning grating 495. The coupled back beam 484 is further redirected bythe tuning grating 495 and coupled through an imaging lens 440. Thetuning grating 495 enables tuning of the coupled back beam 484 to havevariable frequencies. The tuning grating 495 promotes general wavelengthresolution for the ring laser without being too restrictive to therequited wavelength band for the proper phasing of all of the fiberamplifiers arrays 410 of the system 400. At the receiving end 424 of thefiber amplifier array 410, an array of microlenses 428 receives andfocuses the coupled back beam 484 into the fiber amplifier array 410.

The system 400 may also include an isolator 490 so that beams flow inone direction only. The isolator 490 may block ASE and SBS.

FIG. 5 illustrates a fifth exemplary embodiment of the fiber amplifiersystem 500 for passive phasing of fiber amplifiers. The system 500 has acompact design and enables frequency doubling of laser beams, thusallowing the use of different frequencies and wavelengths. The system500 includes an array of fiber amplifiers 510 connected to an array ofmicrolenses 520 at a transmitting end 522. The transmitting end 522 maybe enclosed in an output head 502. The output head 502 may be, forexample, a cylinder that is twenty to twenty-five centimeters long. Thefiber amplifier array 510 emits a beam 582 using the microlens array520.

The output head 502 may include a beamsplitter 585 that redirects asmall portion, such as four percent, of the emitted beam 582 into acollimator 550. The small portion is referred to as a coupled back beam584. The rest of the emitted beam 582 propagates through two lenses 544,546 and a second harmonic generator (SHG) crystal 560, forming an outputbeam 580. As noted above, the line width of the fiber amplifier systemis typically narrow because only a subset of the supported longitudinalmodes lases. The narrow line width enables frequency doubling and Ramanshifting. The SHG crystal 560 may double the frequency of the emittedbeam 582, so that the emitted beam 582 may change from, for example,infrared to visible light as the output beam 580.

The coupled back beam 584 that is coupled into the collimator 550 may betransmitted to an input head 504. The input head 504 may be in acylindrical shape similar to the output head 502. A single-mode fiber515 may be used to transmit the coupled back beam 584. The single modefiber 515 has Gaussian output, enabling beams to propagate longdistances without diverging. Referring to FIG. 5, the single-mode fiber515 transmits the coupled back beam 584 through a collimating lens 542.The lens 542 collimates the coupled back beam 584 onto a tuning grating595. The tuning grating 595 tunes the frequency of the coupled back beam584 and redirects the coupled back beam 584 into an array of microlens528 at a receiving end 524 of the fiber amplifier array 510.

The system 500 may further include an isolator 590 that allows beams totravel in one direction only. The isolator 590 may also block noises,such as ASE and SBS.

FIG. 6 illustrates a sixth exemplary embodiment of the fiber amplifiersystem 600 for passive phasing of fiber amplifiers. The system 600 usesa fiber splitter 628 to replace microlenses at a receiving end. Similarto the system 500, the system 600 includes an array of fiber amplifiers610 connected to an array of microlenses 620 at a transmitting end 622.The transmitting end 622 may be enclosed in an output head 602. Theoutput head 602 may be, for example, a cylinder that is twenty totwenty-five centimeters long. The fiber amplifier array 610 emits a beam682 using the microlens array 620.

The output head 602 may include a beamsplitter 685 that redirects asmall portion, such as four percent, of the emitted beam 682 into atuning grating 695 and a collimator 650. The small portion is referredto a coupled back beam 684. The tuning grating 695 tunes the frequencyof the coupled back beam 684. The rest of the emitted beam 682propagates through two lenses 644, 646 and a SHG crystal 660, forming anoutput beam 680. The SHG crystal 660 may double the frequency of theemitted beam 682, so that the emitted beam 682 may change from, forexample, infrared to visible light as the output beam 680.

With continued reference to FIG. 6, a single-mode fiber 615 may be usedto transmit the coupled back beam 684 to the fiber splitter 628 at areceiving end 624 of the fiber amplifier array 610. The fiber splitter628 may branch one fiber amplifier into a number of fiber amplifiers,allowing the coupled back beam 484 to be fed into a large number ofinput fiber amplifiers at the receiving end 624. Consequently, nomicrolenses are needed at the receiving end 624 of the fiber amplifierarray 610 to receive the coupled back beam 684.

The system 600 may further include an isolator 690 that allows beams totravel in one direction only. The isolator 690 may also block noise,such as ASE and SBS.

With reference back to FIG. 2, the system 200 is modeled in an exemplaryexperiment using Gaussian beam propagation in an optical systemrepresented by an ABCD matrix. The exemplary modeling experimentanalyzes feedback fields emitted by the optical system and calculatesenergies of the coupled back beam 284 that is coupled back into thefiber amplifier array 210.

In the exemplary modeling experiment of the system 200, the array offiber amplifiers 210 emits single-mode Gaussian beams 282 with identicalspot sizes and powers. The exemplary modeling experiment firstcalculates the Gaussian beam emitted from each fiber amplifier andcoupled into each microlens. The exemplary modeling experiment thenpropagates the resulting Gaussian beams 282 to an output coupler, suchas the far-field mirror 250 in FIG. 2, and back into the microlens array220. The fiber amplifier array 210 is at the focal plane of themicrolens array 220. If the Gaussian beam emitted from each fiberamplifier has a waist radius of w₀, then the Gaussian beam 282 coupledback into the microlens array 220 may have a waist radius of w_(f),which is given by, $\begin{matrix}{w_{f} = {w_{o}\sqrt{1 + \left( \frac{\lambda\quad f}{\pi\quad w_{o}^{2}} \right)^{2}}}} & (1)\end{matrix}$where f is the focal length of each microlens.

In the exemplary modeling experiment, the ABCD matrix of the propagationpath from the microlens array 220 and back to the same array is{A,B,C,D}. The total feedback field at the microlens array 220 is givenby, $\begin{matrix}{{{E_{fb}\left( {x,y} \right)} = {\sum\limits_{n = 1}^{N_{x}}{\sum\limits_{m = 1}^{N_{y}}{\frac{q_{o}}{{Aq}_{o} + B}{\mathbb{e}}^{\frac{{- i}\quad\pi}{\lambda}{{(\frac{{Cq}_{o} + D}{{Aq}_{o} + B})}{\lbrack{{({x - {AnT}_{x}})}^{2} + {({y - {AmT}_{y}})}^{2}}\rbrack}}}{\mathbb{e}}^{\frac{{- {\mathbb{i}}}\quad 2\pi}{\lambda}{({{{CnT}_{x}x} + {{CmT}_{y}y}})}}{\mathbb{e}}^{{\mathbb{i}}\lbrack{\frac{\pi\quad{AC}}{\lambda}{({{n^{2}T_{x}^{2}} + {m^{2}T}}}}}}}}}{{where},}} & (12) \\{q_{o} = \frac{{\mathbb{i}\pi}\quad w_{L}^{2}}{\lambda}} & (3)\end{matrix}$and T_(x) and T_(y) are the array period along the x-axis and y-axis,respectively. The coherent and incoherent power intensities are givenby, $\begin{matrix}{I_{coherent} = {{\sum\limits_{n,m}{E_{n,m}\left( {x,y} \right)}}}^{2}} & \left( {4a} \right) \\{I_{incoherent} = {\sum\limits_{n,m}{E_{n,m}}^{2}}} & \left( {4b} \right)\end{matrix}$

In the exemplary modeling experiment, T_(x)=250 μm, w_(L)=60 μm, A=1,B=78 cm, C=0, D=1. w_(L) is the beam field radius (1/e) of the coupledback beam 284. The beam spot radius (1/e) of the emitted beam 282 is 4.4μm. The microlens focal length is 780 μm, resulting in a beam spotradius of 60 μm at the microlens array plane.

FIG. 7 illustrates an exemplary experimental far-field intensity profilefor the fiber amplifier system 400 of FIG. 4. In this exemplaryexperiment, a 2×2 two-dimensional fiber amplifier array 410 is used. Afeedback signal is captured using the beam splitter 485 to direct afraction of the emitted beam 482 into the imaging lens 440 of asingle-mode fiber amplifier. This signal is first amplified by apre-amplifier (shown in FIG. 3) before being split into four beams by a1×4 fiber splitter (shown in FIG. 6). Each beam is fed into one of thefiber amplifiers in the array 410. The far-field intensity pattern isdepicted in the frame captured by a charge coupled device (CCD) camera.The dotted line represents actual experimental intensity slices alongthe x-axis and y-axis. The solid line represents correspondingtheoretical simulation of the experiment. As shown in FIG. 7, theexperimental result closely correlates to the theoretical simulation.The beam quality is estimated to be about 1.03 (M²) corresponding to anenergy spread (RMS) phasing error of about 0.05.

FIG. 8 illustrates a comparison of theoretical and experimentalintensity profile measurement for two coupled fiber amplifiers for thefiber amplifier system 400 of FIG. 4. The measured Strehl ratio for FIG.8 was 0.93, corresponding to a beam quality of 1.04. The high coherenceof this output is indicated by the relatively high visibility of 92%.The coherence length is estimated to be longer than 1 meter. Thebandwidth is estimated to be about 200 MHz. An RF spectrum analyzer isused to measure the mode beats. The frequency separation of longitudinalmode is determined to be about 2.0 MHz, which is consistent with thecalculated value. The fiber amplifiers have an average length of about50 meters to within few centimeters. A refractive index of 1.45 may leadto a mode separation of 2.06 MHz. Theoretical estimates indicate thatabout one hundred neighboring longitudinal modes are present.

In a corresponding experiment, the frequency separation of thelongitudinal modes is about only 2 MHz. The largest expected phasedeviation in the fiber amplifier array depends on the deviation in thelengths of the fiber amplifiers and the frequency shift of thelongitudinal mode with respect to the “resonant mode.” The largest phasevariance, i.e., the variation about a nominal value, is given by,Δφ_(max)=2(Δk _(max))(ΔL _(max))n   (5)where Δφ_(max) and ΔL_(max), are the maximum phase and length variationamong the fiber amplifier array, respectively. The refractive index ofthe fiber amplifiers is represented by n=1.45. In this exemplaryexperiment, the fiber amplifiers have a length variation of 10 cm, andthe maximum acceptable phase variation is 1 radian. Equation (5) thusindicates a bandwidth of ΔV_(max)=170 MHz. The coherence length is givenby c/ΔV_(max)=1.8 meters. The measured coherence length is >1. meters.Since the mode separation is about 2 MHz, the number of longitudinalmodes present is about eighty-five modes, which is close to thetheoretical estimates of about 200 MH.

FIG. 9 schematically shows exemplary resonant modes. The resonant modeshave maximum intensity on axis, and thus typically have the highestfeedback power and thus dominate all other modes.

FIG. 10 is a flow chart illustrating an exemplary method 1000 forpassive phasing of fiber amplifiers. The method 1000 places an opticaldevice, such as a mirror 250, a collimating mirror 360, a beamsplitter485, 585, 685, at a far-field around a central lobe on an optical axisof an array of fiber amplifiers 210, 310, 410, 510, 610 (block 1010).The array of fiber amplifiers 210, 310, 410, 510, 610 emits a beam 282,382, 482, 582, 682 (block 1015). The method then couples a portion ofthe emitted beam 282, 382, 482, 582, 682 back into the array of fiberamplifiers 210, 310, 410, 510, 610 through a coupling hole 230, 330, 430(block 1020 ). A first portion of the emitted beam 282, 382, 482, 582,682 that is in-phase propagates through the coupling hole 230, 330, 430.

While the system and method for passive phasing of fiber amplifiers havebeen described in connection with an exemplary embodiment, those skilledin the art will understand that many modifications in light of theseteachings are possible, and this application is intended to covervariations thereof.

1. A method for passive phasing of fiber amplifiers, the methodcomprising: placing an optical device at a far-field of an array offiber amplifiers; emitting a beam from the array of fiber amplifiers;and coupling a first portion of the emitted beam back into the array offiber amplifiers through a coupling hole, wherein only the first portionof the emitted beam propagates through the coupling hole.
 2. The methodof claim 1, wherein the placing step includes placing the optical devicearound a central lobe on an optical axis of the array of fiberamplifiers, wherein only the first portion of the emitted beam that isin-phase propagates through the coupling hole, and wherein the couplingstep includes coupling two to three percent of the emitted beam backinto the array of fiber amplifiers.
 3. The method of claim 1, whereinthe placing step includes placing the optical device around a centrallobe on an optical axis of the array of fiber amplifiers, wherein onlythe first portion of the emitted beam that is in-phase propagatesthrough the coupling hole, and wherein the coupling step includescoupling four percent of the emitted beam back into the array of fiberamplifiers.
 4. The method of claim 1, wherein the placing step includesplacing the optical device around a central lobe on an optical axis ofthe array of fiber amplifiers, and wherein the emitting step locks thefiber amplifiers so that the fiber amplifiers operate at similarfrequencies.
 5. The method of claim 1, wherein the placing step includesplacing the optical device around a central lobe on an optical axis ofthe array of fiber amplifiers, wherein the array of fiber amplifiersincludes a plurality of fiber amplifiers, wherein the emitting stepcomprises using an array of microlenses connected to the array the fiberamplifiers to emit the beam, and wherein the array of microlensesincludes a plurality of microlenses, each microlens corresponding to asingle fiber amplifier.
 6. The method of claim 1, wherein the placingstep includes placing the optical device around a central lobe on anoptical axis of the array of fiber amplifiers, the method furthercomprising receiving the first portion of the emitted beam using anarray of microlenses connected to the array of fiber amplifiers.
 7. Themethod of claim 1, wherein the placing step includes placing the opticaldevice around a central lobe on an optical axis of the array of fiberamplifiers, the method further comprising receiving the first portion ofthe emitted beam using a fiber splitter connected to the array of fiberamplifiers, wherein the fiber splitter is capable of branching one fiberamplifier into a number of fiber amplifiers.
 8. The method of claim 1,wherein the placing step includes placing the optical device around acentral lobe on an optical axis of the array of fiber amplifiers, themethod further comprising using an isolator to transmit the firstportion of the emitted beam in one direction, wherein the isolatorblocks amplified spontaneous emission (ASE) and stimulated Brillouinscattering (SBS).
 9. The method of claim 1, wherein the placing stepincludes placing the optical device around a central lobe on an opticalaxis of the array of fiber amplifiers, the method further comprisingusing fiber Bragg grating (FBG) reflectors to form double-passamplifiers.
 10. The method of claim 1, wherein the placing step includesplacing the optical device around a central lobe on an optical axis ofthe array of fiber amplifiers, wherein the optical device is a mirror inthe far-field, and wherein the method further comprises: forming atelescope with a second mirror; and outputting a second portion of theemitted beam as a collimated output beam, wherein the second portion ofthe emitted beam is not coupled back into the array of fiber amplifiers.11. The method of claim 1, wherein the placing step includes placing theoptical device around a central lobe on an optical axis of the array offiber amplifiers, wherein the optical device is a collimating mirror,and wherein the method further comprises placing a transmitting end anda receiving end of the array of fiber amplifiers toward each other toform a ring configuration.
 12. The method of claim 11, wherein theplacing step includes placing the optical device around a central lobeon an optical axis of the array of fiber amplifiers, the method furthercomprising: coupling the first portion of the emitted beam to an arrayof pre-amplifiers, wherein the array of pre-amplifiers amplifies thefirst portion of the emitted beam before coupling the first portion ofthe emitted beam to the array of fiber amplifiers; and outputting asecond portion of the emitted beam as a collimated output beam, whereinthe second portion of the emitted beam is not coupled back into thearray of fiber amplifiers.
 13. The method of claim 1, wherein theplacing step includes placing the optical device around a central lobeon an optical axis of the array of fiber amplifiers, wherein the opticaldevice is a beamsplitter, and wherein the method further comprises:separating a transmitting end and a receiving end of the array of fiberamplifiers to form a ring configuration; and redirecting the firstportion of the emitted beam using a tuning grating, wherein the tuninggrating is capable of tuning frequencies.
 14. The method of claim 1,wherein the placing step includes placing the optical device around acentral lobe on an optical axis of the array of fiber amplifiers,wherein the optical device is a beamsplitter enclosed in an output head,and wherein the method further comprises: using a second harmonicgenerator (SHG) crystal to double a frequency of a second portion of theemitted beam, wherein the second portion of the emitted beam is notcoupled back into the array of fiber amplifiers; and using a collimatorto receive the first portion of the emitted beam to be coupled back intothe array of fiber amplifiers.
 15. The method of claim 14, wherein theplacing step includes placing the optical device around a central lobeon an optical axis of the array of fiber amplifiers, the method furthercomprising using a single-mode fiber to transmit the first portion ofthe emitted beam to a tuning grating, wherein the tuning grating iscapable of tuning frequencies.
 16. The method of claim 1, wherein theplacing step includes placing the optical device around a central lobeon an optical axis of the array of fiber amplifiers, wherein the opticaldevice is a beamsplitter enclosed in an output head, and wherein themethod further comprises: using a SHG crystal to double a frequency of asecond portion of the emitted beam, wherein the second portion of theemitted beam is not coupled back into the array of fiber amplifiers;redirecting the first portion of the emitted beam using a tuninggrating, wherein the tuning grating is capable of tuning frequencies;and using a collimator to receive the first portion of the emitted beamto be coupled back into the array of fiber amplifiers.
 17. The method ofclaim 16, wherein the placing step includes placing the optical devicearound a central lobe on an optical axis of the array of fiberamplifiers, the method further comprising using a single-mode fiber totransmit the first portion of the emitted beam to fiber splitterenclosed in a receiving end, wherein the fiber splitter is capable ofbranching one fiber amplifier into a number of fiber amplifiers.
 18. Asystem for passive phasing of fiber amplifiers, comprising: an array offiber amplifiers including a plurality of fiber amplifiers, the array offiber amplifiers emitting a beam; and an optical device placed at afar-field of the array of fiber amplifiers, wherein the optical devicecouples a first portion of the emitted beam back into the array of fiberamplifiers through a coupling hole, and wherein only the first portionof the emitted beam propagates through the coupling hole.
 19. The systemof claim 18, wherein the optical device placed around a central lobe onan optical axis of the array of fiber amplifiers, wherein only the firstportion of the emitted beam that is in-phase propagates through thecoupling hole, and wherein the optical device couples two to threepercent of the emitted beam back into the array of fiber amplifiers. 20.The system of claim 18, wherein the optical device placed around acentral lobe on an optical axis of the array of fiber amplifiers,wherein only the first portion of the emitted beam that is in-phasepropagates through the coupling hole, and wherein the optical devicecouples four percent of the emitted beam back into the array of fiberamplifiers.
 21. The system of claim 18, wherein the optical device isplaced around a central lobe on an optical axis of the array of fiberamplifiers, the system further comprising an isolator that transmits thefirst portion of the emitted beam in one direction, wherein the isolatorblocks amplified spontaneous emission (ASE) and stimulated Brillouinscattering (SBS).
 22. The system of claim 18, wherein the optical deviceis placed around a central lobe on an optical axis of the array of fiberamplifiers, the system further comprising an array of microlensesconnected to the array of fiber amplifiers, wherein the array ofmicrolenses includes a plurality of microlenses, each microlenscorresponding to a single fiber amplifier.
 23. The system of claim 18,wherein the optical device is placed around a central lobe on an opticalaxis of the array of fiber amplifiers, the system further comprising anarray of microlenses that is connected to the array of fiber amplifiersand receives the first portion of the emitted beam
 24. The system ofclaim 18, wherein the optical device is placed around a central lobe onan optical axis of the array of fiber amplifiers, the system furthercomprising a fiber splitter that is connected to the array of fiberamplifiers and receives the first portion of the emitted beam, whereinthe fiber splitter is capable of branching one fiber amplifier into anumber of fiber amplifiers.
 25. The system of claim 18, wherein theoptical device is placed around a central lobe on an optical axis of thearray of fiber amplifiers, the system further comprising fiber Bragggrating (FBG) reflectors connected to the array of fiber amplifiers toform double-pass amplifiers.
 26. The system of claim 18, wherein theoptical device is placed around a central lobe on an optical axis of thearray of fiber amplifiers, wherein the optical device is a mirror in thefar-field, wherein the system further comprises a second mirror to forma telescope with the mirror in the far-field, wherein the telescopeoutputs a second portion of the emitted beam as a collimated outputbeam, and wherein the second portion of the emitted beam is not coupledback into the array of fiber amplifiers.
 27. The system of claim 18,wherein the optical device is placed around a central lobe on an opticalaxis of the array of fiber amplifiers, wherein the optical device is acollimating mirror, and wherein the array of fiber amplifiers areconfigured as a ring with a transmitting end and a receiving end of thearray of fiber amplifiers being positioned toward each other to form thering.
 28. The system of claim 27, wherein the optical device is placedaround a central lobe on an optical axis of the array of fiberamplifiers, the system further comprising an array of pre-amplifiersthat amplifies the first portion of the emitted beam before coupling thefirst portion of the emitted beam to the array of fiber amplifiers. 29.The system of claim 27, wherein the optical device is placed around acentral lobe on an optical axis of the array of fiber amplifiers,wherein the collimating mirror outputs a second portion of the emittedbeam as a collimated output beam, wherein the second portion of theemitted beam is not coupled back into the array of fiber amplifiers. 30.The system of claim 18, wherein the optical device is placed around acentral lobe on an optical axis of the array of fiber amplifiers,wherein the optical device is a beamsplitter, wherein the array of fiberamplifiers are configured as a ring with a transmitting end and areceiving end of the array of fiber amplifiers being separated from eachother to form the ring, and wherein the system further comprises: atuning grating that redirects the first portion of the emitted beam,wherein the tuning grating is capable of tuning frequencies.
 31. Thesystem of claim 18, wherein the optical device is placed around acentral lobe on an optical axis of the array of fiber amplifiers,wherein the optical device is a beamsplitter enclosed in an output head,and wherein the system further comprises: a second harmonic generator(SHG) crystal capable of doubling a frequency of a second portion of theemitted beam, wherein the second portion of the emitted beam is notcoupled back into the array of fiber amplifiers; and a collimator thatreceives the first portion of the emitted beam to be coupled back intothe array of fiber amplifiers.
 32. The system of claim 31, wherein theoptical device is placed around a central lobe on an optical axis of thearray of fiber amplifiers, the system further comprising a single-modefiber that transmits the first portion of the emitted beam to a tuninggrating, wherein the tuning grating is capable of tuning frequencies.33. The system of claim 18, wherein the optical device is placed arounda central lobe on an optical axis of the array of fiber amplifiers,wherein the optical device includes a beamsplitter enclosed in an outputhead, and wherein the system further comprises: a SHG crystal capable ofdoubling a frequency of a second portion of the emitted beam, whereinthe second portion of the emitted beam is not coupled back into thearray of fiber amplifiers; a tuning grating that redirects the firstportion of the emitted beam, wherein the tuning grating is capable oftuning frequencies; and a collimator that receives the first portion ofthe emitted beam to be coupled back into the array of fiber amplifiers.34. The system of claim 33, wherein the optical device is placed arounda central lobe on an optical axis of the array of fiber amplifiers, thesystem further comprising a fiber splitter enclosed in a receiving end,wherein the fiber splitter is capable of branching one fiber amplifierinto a number of fiber amplifiers.
 35. The system of claim 34, whereinthe optical device is placed around a central lobe on an optical axis ofthe array of fiber amplifiers, the system further comprising asingle-mode fiber that transmits the first portion of the emitted beamto the fiber splitter.
 36. A system for passive phasing of fiberamplifiers, comprising: an array of fiber amplifiers including aplurality of fiber amplifiers, the array of fiber amplifiers emitting abeam and configured as a ring with a transmitting end and a receivingend of the array of fiber amplifiers being positioned toward each otherto form the ring; a collimating mirror placed at a far-field of thearray of fiber amplifiers, wherein the optical device couples a firstportion of the emitted beam back into the array of fiber amplifiersthrough a coupling hole, and wherein only the first portion of theemitted beam propagates through the coupling hole; and an array ofpre-amplifiers that amplifies the first portion of the emitted beambefore coupling the first portion of the emitted beam to the array offiber amplifiers.
 37. The system of claim 36, wherein the collimatingmirror is placed around a central lobe on an optical axis of the arrayof fiber amplifiers, wherein the collimating mirror outputs a secondportion of the emitted beam as a collimated output beam, wherein thesecond portion of the emitted beam is not coupled back into the array offiber amplifiers.
 38. The system of claim 36, wherein the collimatingmirror is placed around a central lobe on an optical axis of the arrayof fiber amplifiers, wherein only the first portion of the emitted beamthat is in-phase propagates through the coupling hole, and wherein thecollimating mirror couples two to three percent of the emitted beam backinto the array of fiber amplifiers.
 39. The system of claim 36, whereinthe collimating mirror is placed around a central lobe on an opticalaxis of the array of fiber amplifiers, wherein only the first portion ofthe emitted beam that is in-phase propagates through the coupling hole,and wherein the collimating mirror couples four percent of the emittedbeam back into the array of fiber amplifiers.
 40. The system of claim36, wherein the collimating mirror is placed around a central lobe on anoptical axis of the array of fiber amplifiers, the system furthercomprising an isolator that transmits the first portion of the emittedbeam in one direction, wherein the isolator blocks amplified spontaneousemission (ASE) and stimulated Brillouin scattering (SBS).
 41. The systemof claim 36, wherein the collimating mirror is placed around a centrallobe on an optical axis of the array of fiber amplifiers, the systemfurther comprising an array of microlenses connected to the array offiber amplifiers, wherein the array of microlenses includes a pluralityof microlenses, each microlens corresponding to a single fiberamplifier.
 42. The system of claim 36, wherein the collimating mirror isplaced around a central lobe on an optical axis of the array of fiberamplifiers, the system further comprising a fiber splitter that isconnected to the array of fiber amplifiers and receives the firstportion of the emitted beam, wherein the fiber splitter is capable ofbranching one fiber amplifier into a number of fiber amplifiers.
 43. Thesystem of claim 36, wherein the collimating mirror is placed around acentral lobe on an optical axis of the array of fiber amplifiers, thesystem further comprising fiber Bragg grating (FBG) reflectors connectedto the array of fiber amplifiers to form double-pass amplifiers.